Another option is galvanized steel beds, available in many length / width combos. I bought two from Amazon many moons ago and they still look as good as new. If I remember correctly, they were about $50 apiece. No doubt they’re much more expensive now, but they’re still less expensive than buying fresh lumber. And if $$$ is no object, there are those pricey Birdie’s raised beds featured on many popular Youtube channels.
These seem a good option and can be made into many shapes. I have 6 4x16 foot beds and that seems ample.